Such instruments are conventionally known that measure time difference or distance by receiving light pulses reflected from a target or object to be measured. Generally the amount of received light varies greatly according to the distance to the object to be measured. There have been the following problems. In such a case as when the object to be measured is at a great distance, amplification of received light amount is required. On the other hand, in such a case as when the object to be measured is at a very small distance, light intensity of reflected light pulses in the measurement light path is very high, and as a result, the high intensity remaining as it is causes measurement error due to saturation of received light signals.
Because the change in the intensity of received light signal produces measurement error in the course of processing the received light signal, attempts have been made to provide a reference light path for removing influences of light emission time difference at the light emitting section and of the difference in the received light signal processing circuit. In order to carry out particularly high precision measurements, it is preferable that the received light amount (received light level) is approximately the same in both the measurement light path and the reference light path. Therefore, the following methods are known: a method using a light amount adjusting device such as an ND filter (neural density filter; dimming filter) in the reference light path; and a method in which a plural number of pulses of different intensities are emitted and separately used for a short distance measuring system (using a low reflection mirror) and for a long distance measuring system (Refer to for example Patent Document 1).
[Patent Document 1]
JP-A-2000-206246; par. 0016-0024 and FIGS. 1 to 7